1. Field of the Invention
The present invention relates generally to an improvement in a temperature compensating circuit for all kinds of oscillators, especially crystal oscillator (referred to as TCXO, hereinafter), and more particularly, to a temperature compensating circuit for a oscillator in which circuit's compensation characteristic can be varied easily according to the characteristic of the oscillator and to variations in the characteristic, and in which the compensation characteristic can be expanded to a wide temperature range apart from the reference temperature vicinity.
2. Description of the Prior Art
A temperature compensating circuit for a TCXO is used to keep constant the oscillation frequency of the TCXO independently of temperature. In a prior-art temperature compensating circuit, a D.C. voltage supplied from a constant-voltage supply circuit is divided through a voltage divider of combined resistances including such an element having a temperature-resistance coefficient as a thermistor, and the divided voltage is applied to a variable-capacitance diode connected with a voltage-controlled crystal oscillator (referred to as VCXO, hereinafter), whereby the load capacity of the crystal oscillator is varied according to temperature to thereby compensate for the temperature-frequency characteristic of the oscillator.
However, the temperature-frequency characteristic of the crystal oscillator used in the TCXO is expressed generally by a third-order curve, and the oscillation characteristic is also affected by the respective temperature characteristics of the circuit elements. As a result, many calculating steps are required to determine the constants of the temperature compensating circuit. In addition, to provide proper compensation in response to variations in the characteristic of the oscillator, the calculation of the compensating circuit constants and the measurement for confirmation of the calculation results must be troublesomely repeated.
FIG. 1 shows a basic equivalent circuit of a prior-art TCXO temperature compensating circuit, wherein a basic bridge circuit includes resistors R.sub.1 to R.sub.5. At least one of the resistor R.sub.1 to R.sub.5 must include the equivalent resistance component of a temperature sensitive element, but, in principle, some or all of them may be temperature-dependent.
The resistor R.sub.5 between nodes 3 and 4 corresponds to a combined resistance of an equivalent resistance component of a variable-capacitance diode and a resistance component in an input terminal circuit of a VCXO for frequency change. In the case where there are resistances between the input terminal circuit and ground, these resistances can be included in the resistors R.sub.2 and R.sub.4.
In operation, when a reference source voltage E.sub.1 is applied to a node 1, a potential difference appears between the nodes 3 and 4. The differential voltage is supplied to the VCXO to compensate for the temperature-frequency characteristic of the crystal oscillator and to keep the oscillation frequency constant independently of temperature.
A typical TCXO temperature compensating circuit generally has such a configuration as shown in FIG. 2, in which we can consider that a temperature compensating circuit 2 for a VCXO 1 shown in FIG. 2 is a special case of the counterpart shown in FIG. 1, that is, corresponds to a case where the node 3 is grounded. In this case, it will be readily appreciated that a combined resistance of a resistance component R.sub.T of a thermistor and resistors R.sub.6 and R.sub.7 corresponds to the resistor R.sub.3 in FIG. 1, and that a high-frequency blocking resistor R.sub.8 corresponding to the resistance component of the input terminal of the VCXO and the resistance component of a variable-capacitance diode D are included in the resistor R.sub.5 in FIG. 1.
With the prior-art temperature compensating circuit of such an arrangement, as will be obvious from FIG. 2, it is troublesome to determine the respective constants of the circuit by means of calculation and further the circuit adjustment involves a lot of difficulties.
The present invention is directed to the elimination of such defects in the prior-art TCXO temperature compensating circuit.